World population at the beginning of 1970 was over 3.5 billion. Less than two human generations ago, in 1930, it was 2 billion. About eleven generations ago, 1650—the onset of the modern era—it was only half a billion. In little more than one human generation hence, in the year 2000, world population could easily reach 7 billion and possibly greatly exceed this number. Growth rates have quickened: the world population increased from half a billion in 1650 to 1 billion in 1825, to 2 billion in 1930, and to 3 billion in 1960. If present rates continue, there will be 4 billion human beings by 1975. The acceleration is emphasized by calling attention to the ever shorter period required to double human numbers—175 years between 1650 and 1825, 105 years between 1825 and 1930, and 45 years between 1930 and 1975. The public has become more and more aware of the dramatic rise in the rate of world population growth during the three centuries of the modern era. Rapid growth has been one of three related population phenomena generating acute public concern. The other two are the increasing concentration of the world's people on a relatively small proportion of the earth's surface—a phenomenon better known as urbanization and metropolitanization—and the growing diversity of the peoples who share the same geographic area and, increasingly, the same life space and the same economic, social, and political systems. These three population developments are of relatively recent origin, spanning no more than about three of the perhaps forty thousand centuries that man or a close relative has resided on this planet. Only during the course of the present century have these three interrelated developments combined to generate world, national, and local crises, and to force intensified attention to population problems. During the first three centuries of the modern era, from 1650 to 1950, world population multiplied about fivefold, from 0.5 to 2.5 billion, but over *See the following chapters in Vol. II of this study: Philip M. Hauser, "World Population: Retrospect and Prospect"; Nathan Keyfitz, "Changes of Birth and Death Rates and their Demographic Effects"; and Dudley Kirk, "A New Demographic Transition?"